Arm provides a default address map defining default behaviors for
certain address ranges, which can be overlayed with additional regions
in the MPU. Users may also turn off this background map, so that only
regions explicitly programmed in the MPU are allowed.
This provides a Kconfig so that platforms using a non-standard address
map may disable the background address map and provide their own
explicit MPU regions.
Signed-off-by: Benjamin Gwin <bgwin@google.com>